1. Field of the Invention
The present invention relates to a method and system for controlling creep in an automatic transmission.
2. Related Art
In a vehicular transmission having a starting clutch (a torque transfer mechanism) which uses an intermeshing type transmission to engage and disengage a prime mover (hereinafter referred to as “engine”) and a transmission with and from each other, if a creep torque is to be controlled with a transfer torque of the starting clutch, since the torque transfer is realized in a slipping state of the starting clutch, there arises a great change in the transfer torque due to wear of the starting clutch and due to an increase of temperature caused by the wear. In an effort to avoid this inconvenience there has been proposed a method of controlling the slipping speed of the clutch (Japanese Patent Laid Open No. 7-77226), a method in which a required torque is estimated from both clutch torque and vehicle speed and the transfer torque of the clutch is controlled thereby (Japanese Patent Laid Open No. 2000-186726), and a method in which when creep starts, the engine torque is increased by only a predetermined value or up to a rated value and at the same time the transfer torque of the starting clutch is controlled (Japanese Patent Laid Open Nos. 10-71877 and 4-159873).
In the above conventional method wherein the transfer torque of the starting clutch is increased to control the creep torque, the vehicle load varies according to the state of vehicular running environment (slope and an increase of payload) and there arises a decrease in the number of revolutions of the engine, i.e., engine speed. In this case, in an engine controller there is performed an idling speed control for keeping the engine speed constant, thus giving rise to the problem that there occurs engine vibration as the running load increases and a required creep torque cannot be controlled freely. Further, according to the method wherein the engine torque is increased by a predetermined value or up to a rated value, there sometimes occurs a case where it is difficult to suppress the generation of heat from the starting clutch.